DEGREE OF OXIDATION OF ULTRA-HIGH TEMPERATURE CERAMICS IN THE PRESENCE OF CALCIA-MAGNESIA-ALUMINO-SILICATE (CMAS)

Abstract

As operating envelopes of gas turbine engines (GTEs) expand in a quest to seek higher efficiencies through higher inlet temperatures, the need to protect sensitive engine components has become paramount. Thermal and environmental barrier coatings (T/EBCs) have been employed to protect the base metal from inlet temperatures beyond melting points of most materials. These coatings must be capable of withstanding highly oxidizing environments and environments laden with sand and ash composed of calcia-magnesia-alumino-silicate or CMAS, which has a melting temperature below the standard operating inlet temperatures of GTEs. In a small pool of candidates, ultra-high temperature ceramics (UHTCs) have shown promise with sufficient strength, fatigue resistance, thermal shock resistances, oxidation behavior, and coefficients of thermal expansion close to the metals used in GTE fabrication, but the ability to resist CMAS attack is still under exploration. HfB2, ZrB2, and HEB are among the candidates of UHTCs currently in consideration, but the rate of oxidation and the resulting oxide layers suffer lower resistance than the substrate. Two methods of CMAS application were utilized and the resulting penetration depths and oxidation rates were explored to determine the most resistant of the UHTCs.